Since a parallel robot was first proposed in 1938, a parallel robot has been widely used in various fields of society because of its large rigidity, strong bearing capacity, small error, high precision, small self-weight load ratio, good dynamic performance, and easy control, etc. At present, there is a common parallel robot with 2 degree-of-freedom (DOF), 3 DOF, 4 DOF, or 6 DOF. The parallel mechanism with 3 DOF has symmetric distribution of symmetrical branches and is most researched. It is applied in the fields of machining rotary platform, rehabilitation device, satellite positioning device, and installation device, etc.
For example, professor Feng Gao et al. of Yanshan University proposed an offset output a 3 DOF spherical parallel mechanism with a central spherical hinge (CN101306534), which has a typical mechanism features 3-RRR to achieve three-direction rotation around a fixed center. Professor Zhen Huang of Yanshan University proposes a parallel mechanism with symmetrical structure and no accompanying motion (CN102962840), which can continuous rotate in any axis or any point on the middle symmetry plane of the fixed or movable platform, and effectively avoid the occurrence of accompanying motion. Long Shen of Shanghai University proposes a spherical 3 DOF parallel mechanism antenna structure system (CN101924266), which has a larger working space than the general 3 DOF spherical mechanism. Rongfu Lin et al. of Shanghai Jiaotong University proposed a 3 DOF spherical parallel mechanism with arc-shaped sliding pairs (CN104827463), which uses the arc-shaped sliding pairs, the movable platform rotates in three directions around the arc-shaped midpoint is achieved.
Most kinematic pair configurations of these mechanism branches mostly include RR or RRR. The axes of the kinematic pair intersect at one point to achieve movement of 3 DOF of a movable platform. Lin Rongfu of Shanghai Jiao Tong University uses arc-shaped sliding pair. Thereby, the movable platform is bound by forces from three directions through arc-shaped configuration of the kinematic pair. It can only rotate in three directions around an arc-shaped center. The motion is rather complication and the control precision is not very high.